This application incorporates by reference Taiwanese application Serial No. 90113275, filed on May 31, 2001.
1. Field of the Invention
The invention relates in general to an aluminum (Al) wiring layer, and more particularly to an aluminum wiring layer capable of preventing hillocks and a method of forming the same.
2. Description of the Related Art
As the semiconductor manufacturing of an integrated circuit (IC) with larger scale is required, a substrate may be insufficient to provide an enough area for forming required interconnects for the IC. In order to meet the requirement of the formation of increased numbers of interconnects due to the reduction of metal oxide semiconductors (MOSs) of the IC in sizes, two or more levels of metal layers for interconnects have become a necessary technology adopted in the manufacturing of many ICs. Particularly, for some integrated circuits with sophisticated functions such as microprocessors, four or five levels of metal layers are required to implement interconnections of the components of the integrated circuits. On the other hand, in the manufacturing of a thin-film transistor liquid crystal display (TFT-LCD) panel, thin metal films are employed as electrodes and interconnects, which are also formed in a structure with multiple levels of metal layers.
In a structure with multiple levels of metal layers, there are insulating layers, such as dielectrics, formed between any two of the metal layers in order to prevent an interlayer short circuit from occurring. In addition, a pure metal or an alloy with low electric resistance is suitably used as the material for the metal layers. In general, for examples of pure metals, Al, Cu, Mo, Ta, and W can be used. As examples of alloys with low electric resistance, an aluminum alloy containing one or more selected from the other elements, such as Alxe2x80x94Cu, Alxe2x80x94Cuxe2x80x94Si, Alxe2x80x94Pd, and Alxe2x80x94Nd, is used. Preferably, pure aluminum is employed as the material for metal layers. It is because aluminum has considerable adhesion with the substrate, and considerable etching characteristics in manufacturing as well as low electric resistivity. Besides, the earth contains much aluminum than other metal elements. Thus, aluminum is available and low in cost.
However, it still has disadvantages to use pure aluminum, which has a melting point lower than other metals, as the element for metal layers. Referring to FIG. 1A, it illustrates the deposition of a metal on a glass plate. In the manufacturing of thin film transistors, firstly, crystal particles 104 are formed on a glass plate 102 by the precipitation of metal under relatively low temperature (about 150xc2x0 C.) and grain boundaries 106 are formed between the crystal particles. In fact, the crystal particles will not formed regularly in the same way as shown in FIG. 1A and the regular crystal particles shown in FIG. 1A are for the sake of illustration. Next, annealing is performed so that the increased vibration of the crystal particles by heating at high temperature causes the re-arrangement of the atoms of the crystal particles, thereby reducing defects of the crystal particles and re-crystallizing the crystal particles. After the re-crystallization, inner stress of the crystal particles is rapidly reduced by the reduction of the density of defects such as dislocation. If the annealing temperature is being increased and raises the crystal particles formed in the re-crystallization to a higher energy level exceeding the surface energy among the crystal particles, the crystal particles begin to grow while the smaller ones of them vanish. Consequently, the growth of the crystal particles yields larger crystal particles and the grain boundaries of the smaller crystal particles vanish. Thus, the inner stress of the crystal particles is further reduced to a lower level.
When pure aluminum is used as the wiring layer material, hillock and the like may be produced. FIG. 1B shows the hillock by illustrating the glass plate with pure aluminum as the wiring layer material after annealing. In the annealing, the high temperature causes the thermal expansion of Al crystal particle 104 and glass plate 102. Since aluminum has a greater thermal expansion coefficient than the glass, a substantial compressive stress by the Al crystal particle 104 is applied to the glass plate 102. By this compressive stress, the aluminum atoms move along grain boundary 106 to cause a hillock 110. The hillock and the like, such as the hillock 110, may cause the unevenness of the thickness of the other layers in the subsequent fabrication process. Besides, in the worse case, an interlayer short circuit may occur when a large hillock penetrates an insulting layer (not shown) to be formed between the underlying metal layer and the overlying metal layer, and touches the overlying metal layer.
Hence, it is necessary to solve the problem of hillock in order to use Al as the wiring material. Conventionally, there are three approaches to this problem. The first approach is to use the other element having a high melting point, such as Nd, Ti, Zr, Ta, Si, and Cu, as the wiring material. FIG. 2A shows that crystal particles 204 of an Al alloy formed on a glass plate 202 after annealing. As shown in FIG. 2A, there is no hillock formed among grain boundaries 206 of the crystal particles 204 of the Al alloy. Since the atoms of the additional element of the Al alloy cannot dissolve in Al crystal particles, as the crystal particles 204 grow, the atoms of the additional element move to the grain boundaries 206 and gradually form small particles 210 among the grain boundaries 206. Thus, when Al atoms move along the grain boundaries 206, the small particles 210 hinder the Al atoms from moving above the crystal particles 204, suppressing the formation of hillock.
The second approach is to form a metal layer with high melting point covering the Al crystal particles so as to suppress the growth of hillock. FIG. 2B illustrates a metal layer capping the Al crystal particles. After a metal layer 212 with a high melting point is plated over the Al crystal particles 204, annealing is performed. Since the metal layer 212 works as caps for covering the exits formed by the grain boundaries 206 among the Al crystal particles 204, Al atoms are blocked from forming hillocks along the grain boundaries 206. In addition, there is provided with a variant of the second approach where an Al layer in a single amorphous phase is substituted for the metal layer 212. The term xe2x80x9camorphousxe2x80x9d indicates a non-crystalline state, that is, a state where there is no regulation in the atom array of the interior of a substance. Thus, the Al layer in a single amorphous phase has no crystal particle as a core for the growth of crystal particles and can be formed on the crystal particles 204 for the suppression of the formation of hillock.
In the third approach, an additional metal layer with a thermal expansion coefficient between that of the glass plate and Al is applied as a barrier to suppress the formation of hillock. As shown in FIG. 2C, a metal layer 214 is sandwiched between the glass plate 202 and the Al crystal particles 204. The metal layer 214 is first plated on the glass plate 202 and the Al crystal particles 204 are then formed on the metal layer 214. Besides, the metal layer 213 has a thermal expansion coefficient being greater than that of the glass plate 202 but smaller than that of the Al crystal particles 204. During annealing, the metal layer 214 acts as a buffer against the compressive stress due to a difference in thermal expansion coefficient between the glass plate and Al so as to prevent the Al atoms from moving along the grain boundaries 206 and forming hillocks.
For these three convention approaches to the problem of forming hillocks, it is the first one that is the most effective and usually employed. For example, a Japanese company, Kobelco, provides an Alxe2x80x94Nd alloy as the wiring material for metal layers, which is described in U.S. Pat. No. 6,033,542 to Yamamoto, et al. Nd has a large atomic weight and a high melting point, so that Nd can form small particles to hinder Al atoms from moving along the grain boundaries and forming hillocks. In this way, hillocks do not occur even if the temperature reaches 300xc2x0 C. However, manufacturing cost is increased because Nd is a rare earth element, and it is required to apply a low sputtering rate in order to avoid splashing. Besides, Nd has a high resistivity so that an Alxe2x80x94Nd alloy has a resistivity higher than that of the pure aluminum.
As described above, the use of Al as wiring or electrode material in general semiconductor and liquid crystal display manufacturing is desired so that the study of the prevention of generating hillocks when Al is used therein is of great significant.
It is therefore an object of the invention to provide an aluminum (Al) wiring layer capable of preventing hillocks and a method of forming the same. Under different film formation conditions, a wiring layer with a multiple pure Al layers or with a multiple layers having substantially aluminum for its main component is formed and is capable of preventing the occurrence of hillocks on the surface of the wiring layer.
The invention achieves the above-identified object by providing a wiring layer including at least two metal layers formed on a substrate, wherein each metal layer has either pure Al or substantially Al for its main component. The metal layers include a first metal layer, which is formed on the substrate and includes a plurality of first Al crystal particles, and a second metal layer which is formed on the first metal layer and includes a plurality of second Al crystal particles. The second Al crystal particles of the second metal layer are substantially larger in size and are more densely distributed than the first Al crystal particles of the first Al layer. In this way, for any metal layer of the wiring layer, the closer to the substrate the metal layer is, the smaller in size and less densely distributed the Al crystal particles of that layer are. As a result, the pure Al layers of the wiring layer are capable of preventing hillocks.
The invention achieves the above-identified object by providing a method for forming a hillock-free wiring layer which is formed on a substrate and includes at least two metal layers, wherein each metal layer has either pure Al or substantially Al for its main component. The method includes the following steps. Under a first film formation pressure and a first film formation making power, a first metal layer of the wiring layer is formed on the substrate. Under a second film formation pressure and a second film formation making power, a second metal layer of the wiring layer is formed on the first pure Al layer. Besides, the second metal layer has a plurality of second Al crystal particles which are substantially larger in size than a plurality of first Al crystal particles of the first metal layer, and the second Al crystal particles of the second metal layer are more densely distributed than the first Al crystal particles of the first metal layer. In addition, the first metal layer has a resistance greater than a resistance of the second metal layer. The first and second film formation pressures are fixed in the respective steps and are at least about 4.0 Pa, while the first film formation sputtering power is smaller than the second film formation sputtering power.